1. Field of the Invention
The present invention relates to a method for minimising the phase errors during the driving of an electric motor, and a circuit using the method thereof, particularly but not exclusively for minimising the phase errors during the driving of a brushless motor, driven by prefixed driving signals, such as sinusoidal driving signals.
2. Description of Related Art
A DC brushless motor is a synchronous motor and it has a stator winding, a permanent magnet rotor assembly, and internal or external devices to sense rotor position. The rotor assembly may be internal or external to the stator in the brushless motors.
A characteristic of the brushless motor is that the combination of an inner permanent magnet rotor and outer windings offers the advantages of lower rotor inertia and more efficient heat dissipation with respect other type of electric motors.
Moreover, the elimination of brushes reduces maintenance, increases life and reliability, and reduces acoustic noise and EMI (Electromagnetic Interference).
However, the brushless motor, for its best way of working, needs of a well defined relationship among the driving signals and the rotor position. In fact the sensing devices provide signals for electronically switching or commutating of the stator windings in a proper sequence to maintain rotation of the magnet rotor assembly.
Therefore, the rotor position sensing is essential for proper commutation of the brushless motor and to detect said rotor position a few solutions are used, such as Hall effect switches (using Hall effect sensors) or induced BEMF (Backward Electromotive Force).
Once deduced the rotor position, that is once deduced the velocity of the rotor of the brushless motor, said information will be used to provide some driving sinusoidal signals (or pseudo sinusoidal signals) having a suitable frequency.
Usually, the sinusoidal driving systems are used to minimize the ripples of torque and to minimize the acoustic noise.
As stated before the brushless motor, for its best way of working, needs of a well defined relationship among the driving signals and the rotor position and, in fact, in the known sinusoidal driving systems, said relationship is based on the reading of the previous electric period so as to determine the frequency of the actual driving signal to control the electric motor.
Such a nature of approach, in the case of a steady state condition, that is in the case of a constant velocity of the rotor, does not introduce any phase error, because all the electric periods come in succession, having an equal duration, if in a first approximation the electric motor asymmetries are neglected; but in presence of abrupt accelerations or decelerations, the phase error between the driving signal and the rotor position is particularly evident and it causes an increasing of the acoustic noise, an increasing of the vibrations and an increasing of the EMI.
Such of problems are particularly palpable in applications wherein the motor is connected to a load having a low inertia, such as, for example, in Compact Disc (CD) or Digital Versatile Disk (DVD) or similar.
With reference to FIG. 1, a relationship between a driving signal and the rotor position in the case of steady state according to the prior art is shown.
In fact, as shown in such a FIG. 1, a driving signal 1, or it can be also called as phase current, and a rotor position signal 2 in the condition of constant velocity of the rotor are depicted.
In fact, the rotor position signal 2 states that the electric motor (not shown in FIG. 1) has a constant frequency or a constant angular velocity, that is its electric periods Te(n), indicated as Te(n), Te(nxe2x88x921), Te(nxe2x88x922) etc., have the same duration.
In fact, the electric periods are equal each other, by the following relationship: Te(n)=Te(nxe2x88x921)=Te(nxe2x88x922), etc.
In this embodiment, the electric periods Te(n), Te(nxe2x88x921), Te(nxe2x88x922) of the electric motor are deduced between two rising edge of the rotor position signal 2 and they are used to compute the period Ts(n) of the driving signal 1.
Therefore, the period Ts(n) represents the period forced by the control circuit to the electric motor.
In fact, one of the driving technique to control the electric motor, foresees that the period Ts(n) of the driving signal 1 is based on the reading of the previous electric period Te(nxe2x88x921) of the electric motor deduced by the rotor position signal 2, that is the system, in function of the reading of the previous electric period of the electric motor, provides the actual driving signal 1.
In other word, the period Te(nxe2x88x922) of the electric motor is used to compute the period Ts(nxe2x88x921) of the driving signal 1, the period Te(nxe2x88x921) is used for Ts(n) and so on.
This embodiment, as heretofore stated, does not arise distortions only if the electric periods Te(n), Te(nxe2x88x921), Te(nxe2x88x922) etc., have the same duration.
In the case of abrupt accelerations or decelerations some distortions are induced because the measure of the electric period Te(n) changes continuously in the time. Therefore the system, basing its decision for the generation of the driving signal period Ts(n) on the previous electric period Te(nxe2x88x921), will make a phase error between the actual driving signal, that is the period forced by the control circuit, and the ideal driving signal.
In fact, as is shown in FIG. 2, wherein the same relationship of FIG. 1 in the case of abrupt acceleration according to the prior art is shown, and as is shown in FIG. 3, wherein the same relationship of FIG. 1 in the case of abrupt deceleration according to the prior art is shown, it is possible to note in which way the driving signal 1 is modified.
In particularly, referring to the FIG. 2, the system forces an estimated period Test(n) of the driving signal 3 in function of the previous electric period Te(nxe2x88x921) of the rotor position signal 4.
It is to be noted that in this specific embodiment the rotor position signal 4 is equal to the electric period of the motor.
It is to be noted also that the estimated period Test(n) is not completely applied to the brushless motor, because the angular velocity of the rotor is incrementing instant by instant and therefore the rising edge of the rotor position signal 4 arrives with few instants before the predicted instant.
In this way, the estimated period Test(n) is bigger than the actual driving signal period Ts(n), by a factor xcex94acc(t).
In fact, with the factor xcex94acc(t) is depicted the difference between the estimated driving signal period Test(n), and the actual electric period Te(n).
In particularly, referring to the FIG. 3, the system forces an estimated period Test(n)xe2x80x2 of the driving signal 6 in function of the electric period Te(nxe2x88x921)xe2x80x2 of the rotor position signal 7.
It is to be noted that in this specific embodiment the rotor position signal 7 is equal to the electric period of the motor.
It is to be noted also that the estimated period Test(n)xe2x80x2 can not complete the actual electric period Te(n)xe2x80x2, because the angular velocity of the rotor is decrementing instant by instant and therefore the rising edge 8 of the rotor position signal 7 arrives few instants after the predict instant.
In this way, the estimated period Test(n)xe2x80x2 is lower than the actual driving signal period Ts(n)xe2x80x2, by a factor xcex94dec(t).
In fact, with the factor xcex94dec(t) is depicted the difference between the actual electric period Te(n)xe2x80x2 and the actual driving signal Test(n)xe2x80x2.
Referring to the FIG. 3, it is to be noted that the driving signal 6 ends its cycle xcex8, made by of 360xc2x0 degrees, before the arrival of the rising edge 8 of the rotor position signal 7, and in the specific embodiment, the committed phase error xcex1, during the driving of the brushless motor, can achieve high values, such as, for example, xcex1=90xc2x0 degrees.
The FIGS. 2 and 3 are related to a system wherein the generation of the driving signals 3 or 6 continues unchanged until the arrival of the next rising edge 5 or 8 of the respective rotor position signal 4 or 7.
Another embodiment, well known to a skilled person, is that in which, the driving signal is produced for a maximum duration of 360xc2x0 degrees.
In view of the state of the art described, it is in object of the present invention to solve the aforementioned problems, particularly to solve the phase errors committed during the driving of the brushless motors driven by means of prefixed driving signals in presence of abrupt accelerations or decelerations.
According to the present invention, such object is attained by a method for minimising the phase errors during the driving of an electric motor, having a stator winding, a permanent magnet rotor assembly, and devices able to sense a rotor position, characterised by comprising the following steps: a) generating of a rotor position signal, by means of said devices able to sense said rotor position; b) detecting at least two information from at least two edges of said rotor position signal inside a measure period; c) generating a driving signal, in function of said at least two information inside the measure period, so as to follow the rotor velocity.
According to the present invention, such object is also attained by a circuit for minimising the phase errors during the driving of an electric motor, having a stator winding, a permanent magnet rotor assembly, detecting means able to detect a rotor position, storing means able to store a number of samples of an ideal driving profile, addressing means able to address one of that stored samples in said storing means, characterised in that said detecting means output a rotor position signal, used for pointing to said stored samples of said ideal driving profile in said storing means by means of said adding means and used by means of a frequency multiplier means so as to provide a scanning frequency signal able to scan said samples in said storing means.
Thanks to the present invention it is possible to realise a method and a circuit able to reduce the acoustic noise, the vibrations and the EMI of a brushless motor in presence of abrupt accelerations or decelerations.